Episomal expression vector for human gene therapy

ABSTRACT

Episomal plasmids containing a papovavirus origin of replication and a papovavirus large T antigen mutant form are shown to replicate episomally in human cells, and yield levels of gene expression proportional to their episomal copy number. In conjunction with liposomal or receptor-mediated delivery systems, papovavirus-derived episomal plasmids provide an alternative vector for gene therapy, particularly when utilizing strategies requiring high levels of gene expression.

The work leading to this invention was supported in part by Grant No.P30 CA43703 from the National Institutes of Health. The U.S. Governmentmay retain certain rights in this invention.

This application is a division of application Ser. No. 08/151,387, filedNov. 12, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to papovavirus-derived episomes thatreplicate efficiently in mammalian cells, yielding stable transfectantshaving a high episomal copy number and expressing encoded genes at highlevels. Papovavirus-derived episomes may be useful in gene therapystrategies to modulate the growth of bladder carcinoma cells.

2. Review of Related Art

One approach to gene therapy of human cancer cells is to introducevectors expressing antisense sequences to block expression of dominantoncogenes and growth factor receptors. However, high-level expression ofthe oncogenes requires comparable levels of antisense expression, whichpresents a considerable technical obstacle, particularly when usingexpression vectors having a limited potential for achieving multiplecopies in stable transfectants. Human cells transduced by retroviralvectors have only one or several copies of integrated retrovirus instable transfectants. In contrast, hundreds of copies of episomalplasmids can accumulate in stable transfectants because these vectorsreplicate extrachromosomally. One method to express high levels ofantisense transcripts is to utilize episomal plasmid vectors than canreplicate extrachromosomally in human cells.

Attempts to produce episomal vectors that will replicate in some typesof human cells are reported by the literature. Episomal plasmids havebeen developed from several DNA viruses, including bovine papillomavirus (BPV) (Sarver, et al., 1981, Mol. Cell. Biol., 1: 486-496; DiMaio,et al., 1982, Proc. Natl. Acad. Sci., U.S.A., 97: 4030-4034), SV40(Tsui, et al., 1982, Cell, 30: 499-508), Epstein-Barr virus (EBV)(Yates, et al., 1985, Nature, 313: 812-815; Margolskee, et al., 1988,Mol. Cell. Biol., 8: 2837-2847; Belt, et al., 1989, Gene, 84: 407-417;Chittenden, et al., 1989, J. Virol., 63: 3016-3025), and BK virus (BKV)(Milanesi, et al., 1984, Mol. Cell. Biol., 4: 1551-1560). Each of theseepisomal plasmids contains a viral origin of DNA replication and avirally encoded early gene that trans-activates the viral origin andallows the episome to replicate in the transfected host cell.

Although EBV-based episomes have been used to efficiently screen cDNAlibraries, the EBV system has limited applications to non-lymphoid celltypes (Vidal, et at., 1990, Biochim. Biophys. Acta 1048: 171-177)), andthe EBV replicon is not active in many cell types. Additionally, EBNA-1is one of several EBV latent genes that immortalize human lymphocytes,and transfection of the EBV-negative BJAB lymphoma cell line by EBNA-1induces soft agar growth, indicating transformation of the cells.(Konoshita, 1990, Hokkaido Igaku Zasshi, 65: 362-375)

Furthermore, stable transfection efficiencies for EBNA-1 negative celllines transduced by EBV episomal plasmids encoding EBNA-1(transactivator) and ORI-P (EBV DNA origin) are low, not significantlybetter than non-episomal plasmids (Yates, et al., 1985; Vidal, et at.,1990. However, if EBNA-1 is expressed in cells prior to transfection,then a subsequent transfection with a plasmid containing ORI-P and aselectable marker can yield stable transfection efficiencies of up to10% (Margolskee, et al., 1988; Belt, et al., 1989; Yates, et al., 1984,Proc. Natl. Acad. Sci. U.S.A. 81: 3806-3810; Lutfallia, et al., 1989,Gene 76: 27-39). Comparable results have been noted in a related systemof COS cell clones expressing high levels of SV-T, which permitefficient replication of SV40 origin-containing plasmids in transienttransfectants (Tsui, et al., 1982; Rio, et al., 1985, Science 227:23-28; Chittenden, et al., 1991, J. Virol. 65: 5944-5951).

In the COS cell system, however, episomal replication can proceed in arunaway fashion, resulting in up to 10⁴ episomal copies by 48 hoursafter transfection. Despite efficient episomal replication in transienttransfectants, low stable transfection efficiencies have been noted inthese studies (Chittenden, et al., 1991; Roberts, et al., 1986, Cell,46: 741-752). Presumably, most transient transfectants die secondary toepisome-mediated cell death (Chittenden, et al., 1991; Roberts, et al.,1986).

However, transfection of COS cells by SV40 DNA origin-containingplasmids does produce stable transfectants having episomal plasmids(Tsui, et al., 1982), and it may be possible to control runaway episomalreplication by a variety of strategies, including use of replicationcontrol regions from other viruses. For example, runaway episomalreplication in COS cell clones can be controlled by use of plasmidscontaining the SV40 DNA origin and regions of the bovine papilloma virus(BPV) replicon (Roberts, et al., 1986; Roberts, et al., 1988, Cell 52:397-404). These studies have identified two BPV sequences (NCOR I andNCOR II) that modulate runaway SV40 episomal replication in transienttransfectants, and a third trans-suppressing factor encoded by 5'sequences in the E1 open reading frame. Hybrid plasmids encoding theSV40 DNA origin and a 2113 bp EcoR1 fragment of BPV have substantiallyhigher stable transfection efficiencies than pSV-NEO (Roberts, et al.,1986). A DNA homology search failed to identify similar NCOR sequencesin the BKV or SV40 replicon.

Thus, there remains a need for vectors which will replicate episomallyin a controlled fashion in mammalian cells for gene therapyapplications. In particular, there is a need for vectors that willreplicate episomally in human cells without transforming the cells.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a vector which willreproduce episomally in a mammalian cell without transforming the cell.

It is another object of this invention to provide a method for genetherapy whereby a foreign gene, encoded on a vector that replicatesepisomally in high copy number without transforming transfected cells,is expressed in a mammalian cell transfected by the episomal vector.

It is still another object of this invention to provide a mutant form ofpapovavirus large T antigen that is replication-competent andtransformation-negative.

In order to achieve these and other objects, the present inventionprovides a mammalian vector which is replication-competent andtransformation-negative, the vector comprising at least one papovavirusorigin of replication, preferably the origin from SV40 or BK virus, anda DNA sequence encoding a mutant form of papovavirus large T antigenwhich contains a replication-competent binding site for the origin ofreplication but which is negative for binding to at least one ofwild-type p53 or retinoblastoma tumor suppressor (RB) gene products,preferably both, the DNA sequence being operatively linked to ahomologous or heterologous promoter. In alternative embodiments of thevector, the DNA sequence encoding a mutant form of papovavirus large Tantigen is operationally linked either to a papovavirus early promoter,to a promoter which is inducible, or to a promoter which is underhormonal control.

In another embodiment, this invention provides a method of expressing aforeign gene in a mammalian cell comprising transfecting the mammaliancell with a replication-competent, transformation-negative vectorcomprising at least one papovavirus origin of replication, a first DNAsequence encoding a mutant form of papovavirus large T antigen whichcontains a replication-competent binding site for the origin ofreplication but which is negative for binding to at least one ofwild-type p53 or retinoblastoma tumor suppressor gene products, the DNAsequence being operatively linked to a first promoter, and a second DNAsequence encoding the foreign gene operatively linked to a secondpromoter; and expressing the foreign gene in the transfected cell. Inpreferred embodiments of this method, the papovavirus origin ofreplication is either the BK virus origin of replication or the SV40origin of replication, and the mutant form of papovavirus large Tantigen is a mutant SV40 large T antigen that binds to both SV40 and BKvirus origins of replication but is negative for binding wild-type p53and also negative for binding to retinoblastoma tumor suppressor geneproduct. In alternative embodiments, the mammalian cell is transfectedby the vector in vitro, then the cell is introduced into a mammal andthe foreign gene is expressed in vivo, or the vector is administered toa mammal and cells of the mammal are transfected in vivo, the foreigngene being expressed by these cells.

In another embodiment, the invention provides a DNA sequence encoding amutant form of SV40 large T antigen which contains areplication-competent binding site for SV40 origin of replication but isnegative for binding to wild-type p53 and is also negative for bindingto retinoblastoma tumor suppressor gene product. In a preferredembodiment, residue 107 of the mutant form of SV40 large T antigenencoded by the DNA sequence is lysine and residue 402 is glutamic acid.

A highly efficient episomal expression vector that replicatesextrachromosomally in human cells has been developed. We havedemonstrated that replication-competent, transformation-negative SV40large T antigen mutants can successfully drive replication of plasmidscontaining the SV40 DNA origin or BK virus origin of DNA replication. Apreferred vector is derived from BK virus (BKV), a small DNA virushaving significant homology to SV40. The properties of BKV episomescharacterized in stable bladder carcinoma cell line transfectants haveshown that these vectors replicate extrachromosomally for at least 5months; achieve a high stable copy number (150) without inducingepisome-mediated cell death; have a very low rate of integration;transcribe genes in proportion to their copy number; are efficientlytransferred to daughter cells during cell division; can be shuttled fromHirt supernatant DNA to bacteria; and even persist in bladder celltransfectants for several months without selection pressure. Theseproperties demonstrate the feasibility of using this vector system totransfer genes to human cells.

This invention makes a significant advance in episomal vector technologyby developing replication-competent, transformation-negative mutants ofpapovavirus large T antigen to drive replication of plasmids containingpapovavirus origins of DNA replication, such as the SV40 or BKV DNAorigins. Since replication-competent, transformation-negative mutantsfor other DNA origin transactivators (such as EBNA-1) are not currentlyavailable, this episomal expression system has the unique feature ofpermitting efficient episomal replication without induction oftransforming properties in the host cell. This advance enablesdevelopment of safe and efficient episomal vectors for human genetherapy applications. This episomal vector system may also havewidespread in vitro applications, such as development of a cancer tumorprogression assay, and has the potential to significantly advancedevelopment of efficient episomal vector systems for cDNA librarycloning and in vitro expression of heterologous genes in human cells.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A and B show Southern analysis of HT-1376 cells stably transfectedwith the BKV episomal vector pRP-cneoX. Cells were evaluated following71 days of G418 selection. Panel A. Hirt DNA from 3×10⁵ cells was loadedin lanes 1-4 and digested with restriction enzymes as indicated.Increasing amounts (50-400 pg) of BamHI-digested pRP-cneoX plasmid wereloaded in lanes 5-9. Panel B. Total cellular DNA (10 pg) from these sameHT-1376/pRP-cneoX stable transfectants was digested with BamHI (lane 1).In lane 2 is 500 pg of BamHI-digested pRP-cneoX plasmid. Hybridizationprobe in both panels was ³² P-labelled pRP-cneoX.

FIGS. 2A-2B show expression of neomycin resistance gene mRNA in HT-1376cells stably transfected with BKV episomal (pRP-cneoX, lane 1) andnon-episomal (pSV2NEO, lanes 2-4) expression vectors. Hybridizationprobes: neomycin resistance gene (Panel A), β-actin (Panel B).

FIGS. 3A-3B show that pRP-cneoX persists as an episomal plasmid inHT-1376 cells following withdrawal of selection pressure. Southernanalysis of BamHI-digested Hirt supernatant DNA from 3×10⁵ HT-1376pRP-cneoX transfectants grown in the presence (71 days, lane 1; 122days, lane 2) or absence (16 days, lane 3; 34 days, lane 4; 47 days,lane 5; 64 days, lane 6) of G418. Hybridization probes were ³² P-labeledpRP-cneoX (panel A) or a 343 bp ³² P-labeled BamHI fragment of mousemitochondrial DNA (ND1) obtained from pKSU1, a derivative of pAM1(Martens, et al. 1979, J. Mol. Biol., 135: 327-351) (panel B).

FIGS. 4A-4B show the persistence of neomycin resistance gene expressionin HT-1376 pRP-cneoX transfectants following withdrawal of selectionpressure. Northern blot analysis of 20 μg of RNA from HT-1376transfectants in the presence (71 days, lane 1; 126 days, lane 6; 156days, lane 7) or absence (16 days, lane 2; 34 days, lane 3; 47 days,lane 4; 64 days, lane 5) of G418. Hybridization probes were a ³²P-labeled BamHI-HindIII fragment of pSV2NEO containing coding sequencesof the neomycin resistance gene (A) or ³² P-labeled β-actin plasmid,pHFβA-1 (Gunning, et al., 1983) (B).

FIG. 5 shows the location of point mutations in replication competent,transformation negative SV40 large T antigen (SV-T) mutants. The p53 andRB binding characteristics of 107/402-T, indicated in parenthesis, arepredicted results.

FIG. 6 shows Western blot analysis of single cell clones of 5637 cellsstably transfected with SV-T or 107-T. Shown are clones of 5637 cellstransfected with pRc/CMV.SV-T (lanes 1-3) or pRc/CMV.107-T (lanes 4-7).Blot was developed using anti-T antigen monoclonal antibody pAB 416 anda chemi-luminescent development system (Amersham). 40 μg of lysate wereloaded per lane.

FIG. 7. Southern blot analysis demonstrating that 107-T drivesextrachromosomal replication of a plasmid (pSV2CAT containing the SV40origin of DNA replication. 107-T 5637 clones C10 (no detectableexpression) and E1 (high level expression) were transfected withpSV2CAT, and Hirt supernatant DNA was prepared approximately 4 daysafter transfection. Hirt supernatant DNA from approximately 5×10⁵ cellswas loaded per lane, and evaluated before and after digested by DpnI, asindicated above. Hybridization probe was ³² P-labelled pSV2CAT.

DETAILED DESCRIPTION OF THE INVENTION

We have identified an episomal vector that efficiently replicates intransformed transitional epithelial cells (e.g., HT-1376 bladdercarcinoma cell line). The vector (pRP-cneoX) contains a marker geneunder control of the SV40 early promoter and a 3.2 kb segment of BKVwhich includes the BKV origin of replication and the BKV large T antigenunder control of the BKV early promoter. Whereas the EBV episomalelement was not active in HT-1376 transient transfectants, BKV episomesreplicated extrachromosomally in these cells. More importantly, BKVepisomes can replicate efficiently in HT-1376 cells without any apparentcellular toxicity, resulting in a high copy number of the episome instable transfectants.

The copy number of BKV episome pRP-cneoX in HT-1376 cells stablytransfected with this construct was approximately 150 copies per cell(see Example 1). This copy number compares to approximately 10-50 copiesof EBV-derived episomes in lymphoblastoid cell lines and 10-80 copies ofbovine papilloma virus-derived episomes in murine C127 cells (Sarver, etal., 1981; DiMaio, et al., 1982; Yates, et al., 1985). The high copynumber of pRP-cneoX in HT-1376 transfectants is likely responsible forthe efficient vertical transfer of pRP-cneoX to the progeny of theseHT-1376 transfectants over multiple generations. The soft agar cloningefficiencies of HT-1376 cells transfected with either integrating vectorpSV2NEO or pRP-cneoX, and plated in the presence or absence of G418,were essentially identical. These data indicate that episomal transferof the neomycin resistance gene to daughter cells was as efficient aswhen this gene is integrated into HT-1376 genomic DNA. This result wasnot unexpected, since the probability that a given daughter cell wouldnot contain at least one copy of the episome would be very low assumingrandom partitioning of the large number of plasmid copies duringcellular division.

Our data demonstrate that a BKV episomal expression vector can producevery high levels of transcription of a transfected gene in HT-1376cells. There was approximately a 20-fold increase in the steady-statelevel of neomycin resistance gene expression in pRP-cneoX transfectantscompared to transfectants which had 5 integrated copies of pSV2NEO.Since the neomycin resistance gene is transcriptionally regulated by theSV40 early promoter in both constructs, these data demonstrate that BKVepisomal vectors can produce significantly higher levels of expressionof a transfected gene that plasmid vectors that must integrate into thehost cell genome to produce stable transfectants. This difference ispresumably due in pan to the higher copy number of pRP-cneoX (150copies) compared to pSV2NEO (5 copies) in HT-1376 transfectants.

Comparison of Episomal Vectors

BKV-derived episomes have several properties that are distinct from EBV,BPV, and SV40-derived episomes. Despite the significant amino acidhomology between the large T antigens from BKV and SV40 (Mann, et al.,1984, Virol., 138: 379-385), BKV episomes can yield stable, viabletransfectants whereas SV40-based episomes replicate to such a high copynumber that cell death typically ensues (Tsui, et al., 1982; Roberts, etal., 1986). This result may be due, in part, to differences in the levelof T antigen present in these transfectants, characteristics of the DNAorigins from these viruses, or presence of cis-regulatory sequences inthe BKV episome that regulate DNA replication, as has been described incomposite SV40-BPV-derived episomes (Roberts, et al., 1986; Hambor, etal., 1988, Proc. Natl. Acad. Sci. U.S.A., 85: 4010-4104).

Significantly, BKV episomes appear to replicate once per cell cycle instable transfectants, because the pRP-cneoX copy number reaches a stableplateau of approximately 150 copies per cell. Stable copy number is alsocharacteristic of EBV and BPV-derived episomes, which can similarlyyield viable, stable transfectants, albeit at lower copy number. Incontrast to EBV-derived episomes (Yates, et al., 1984; Hambor, et al.,1988), however, the copy number of BKV episomes is maintained atunreduced levels after 2 months of growth in the absence of selectionpressure. Fluctuations in pRP-cneoX copy number during the time courseof G418 withdrawal (shown in FIG. 3) presumably represent a dynamicinterplay between factors predisposed to maintain the presence ofepisomes (such as efficient episomal replication during the cell cycleand potential growth advantages present in cells expressing BKV large Tantigen) and factors that may reduce episomal copy number (such asunequal partitioning of the episome during cell division, or destructionby cellular nucleases). Comparable to BKV episomes, BPV episomes canalso be maintained at stable copy numbers in unselected, transformedC127 transfectants (Sarver, et al., 1981; DiMaio, et al., 1982).However, the higher copy number of BKV episomes in unselectedtransfectants is an advantage in strategies to utilize these episomesfor gene therapy.

I. Definitions

In describing the present invention, the following terminology is usedin accordance with the definitions set out below.

A "heterologous" region or domain of a DNA construct is an identifiablesegment of DNA within a larger DNA molecule that is not found inassociation with the larger molecule in nature. Thus, when theheterologous region encodes a mammalian gene, the gene will usually beflanked by DNA that does not flank the mammalian genomic DNA in thegenome of the source organism. Another example of a heterologous regionis a construct where the coding sequence itself is not found in nature(e.g., an intron-free coding sequence (cDNA) where the genomic codingsequence contains introns, or synthetic sequences having codonsdifferent than the native gene). Allelic variations or naturallyoccurring mutational events do not give rise to a heterologous region ofDNA as defined herein.

A DNA "coding sequence" is a DNA sequence which is transcribed andtranslated into a polypeptide in vivo when placed under the control ofappropriate regulatory sequences. A polyadenylation signal andtranscription termination sequence will usually be located 3' to thecoding sequence. A "promoter" is a DNA regulatory region capable ofbinding RNA polymerase in a cell and initiating transcription of adownstream (3' direction) coding sequence. A coding sequence under thecontrol of a promoter in a cell is transcribed by RNA polymerase afterthe polymerase binds the promoter, the coding sequence being transcribedinto mRNA which is then in turn translated into the protein encoded bythe coding sequence.

"Transfection" of a cell occurs when exogenous DNA has been introducedinside the cell membrane. "Transformation" occurs when a cell populationfrom primary cells or a cell line that only undergoes a finite number ofdivisions becomes immortalized, or when an immortal cell line acquiresadditional tumorigenic properties. Transformation can be detected by,for example, the ability of the transformed cell to form clones in softagar or to form tumors in nude or SCID mice. A "clone" is a populationof cells derived from a single cell or common ancestor by mitosis.

A "replicon" is any genetic element (e.g., plasmid, chromosome, virus)that functions as an autonomous unit of DNA replication in vivo.

A "vector" is a replicon, such as plasmid, phage or cosmid, to whichanother DNA segment (a heterologous segment) may be attached so as tobring about the replication of the attached segment.

An "episome" is a low molecular weight DNA molecule that resides in acell separated from the cell's chromosome(s). Episomes replicateindependently of mitotic replication of the chromosomes, beingtransmitted to daughter cells as part of the random reassortment ofcellular contents during cell division. "Copy number" is the number ofduplicate DNA molecules existing in an individual cell as episomes or isthe number of duplicate sequences in the genome. Bacterial episomes areusually called plasmids.

"Foreign genes" are genes that are not found in the genome of theindividual host cell. Foreign genes may be from the same species as thehost or from different species. Where this invention describestransfection of a cell using DNA containing a foreign gene with theintent that the foreign gene will be expressed in the cell, the DNAwill, of course, contain any control sequences necessary for expressionof the foreign gene in the required orientation for expression.

Two DNA sequences that are substantially homologous can be identified bytheir ability to hybridize with each other in a Southern hybridizationexperiment, for example, under stringent conditions as defined for thatparticular system. Defining appropriate hybridization conditions iswithin the skill of the art. See e.g., Maniatis et al., supra; DNACloning, vols. 1 and II supra; Nucleic Acid Hybridization, supra.

II. Description of the Vector

A. Papovaviruses

Papovaviruses are DNA viruses with double-stranded, covalently closed,circular genomes of approximately 5000 bp and icosahedral capsidscontaining three viral proteins. The papovaviruses infect a variety ofhosts, including humans (BK virus and JC virus), monkeys (simianvacuolating virus (SV40) and lymphotropic papovavirus), baboon (simianagent 12), mouse (polyoma virus and K virus), hamster (hamsterpapovavirus), rabbit (rabbit kidney vacuolating virus), and budgerigar(budgerigar fledgling disease virus). These viruses have been judged tobe related based on nucleotide sequence comparisons.

The viral genome is divided into early and late transcription regions,and contains a single origin of replication. Transcription begins frompromoters near the origin of replication and proceedsbidirectionally--one direction for early transcripts and the otherdirection for late transcripts. The late transcriptional region encodescoat proteins (VP1, VP2, and VP3). The early transcriptional regionencodes the T antigens, particularly the large T antigen which functionsin viral DNA replication. The large T antigen also down-regulates earlytranscription by binding to viral DNA near the early promoter, activatescellular genes involved in DNA synthesis, and transforms primary cellsin tissue culture.

Viral DNA replicates in the nucleus as "minichromosomes," but viral DNAcan replicate many times in a single cellular S phase. Viral DNAreplication is initiated by the large T antigen, independently of itsstimulation of cellular DNA synthesis. The large T antigen binds toviral DNA in the neighborhood of the origin of replication and unwindsthe DNA helix, which is required for viral DNA replication. New viralDNA is then synthesized by cellular enzymes.

B. Episomal Amplification Cassette

To provide enhanced expression in gene therapy applications, episomalvectors must replicate extrachromosomally without transforming thetransfected cell. This invention provides a replication cassette forsuch containing the essential elements of papovavirus replication. Thereplication cassette (or episomal amplification cassette) contains 1) apapovavirus origin of DNA replication (ORI); 2) a replication-competent,transformation-negative mutant form of the papovavirus large T antigen;and 3) a promoter to drive expression of the mutant T antigen. When thereplication cassette of this invention is coupled with other DNAsequences in a circular DNA molecule, the DNA molecule will bereplicated episomally by mammalian cells after transfection.

The initial BKV episomal vectors reported by Milanesi, et at. (1984),contained a 3.2 kb fragment of BKV including the origin of DNAreplication and the BKV large T antigen transcriptionally regulated bythe BKV early promoter. As taught below and exemplified in the Examples,the BKV expression system may be modified according to this invention,so that it does not induce soft agar growth in nontumorigenic cells, yetretains the ability to replicate extrachromosomally. The components ofthe replication cassette will be selected according to the followingcriteria and assembled as described below.

1. Origin of Replication

The origin of replication in the replication cassette is selected fromORI sequences of one of the papovaviruses. DNA replication initiated atthese loci is sensitive to control by the large T antigen of the samevirus, and to a similar or lesser extent by large T antigen of otherpapovaviruses.

In the presence of a compatible large T antigen, the papovavirus originwill drive episomal replication. The origin/large T antigen combinationshould be tested to determine whether they drive replication of theepisome. One simple test for replication competency is to transfect apopulation of cells which express the large T antigen mutant proposedfor the replication cassette with a vector containing the proposedorigin of replication and then monitor the transfected cells forsynthesis of episomal DNA by Southern blot (see, e.g., Example 6C).

Particularly preferred is the BKV origin, which has been demonstrated todrive episomal replication with either BKV large T antigen (BK-T) orSV-T. Other preferred origins are those that drive replication inprimates, including SV40, JC virus, lymphotropic papovavirus, and simianagent 12. Any papovavirus origin of replication that can be shown todrive episomal replication in human cells will be suitable for thereplication cassettes of this invention.

The BKV replicon is active in the HT-1376 bladder carcinoma cell line,whereas the Epstein-Barr virus (EBV) replicon is not functional in thesecells. BKV has a trophism for human uroepithelial cells (Arthur, et al.,1986, N. Engl. J. Med., 315: 230-234), and an episomal vector derivedfrom BKV will replicate efficiently in human bladder carcinoma celllines. Hybrid SV40/BK virus-derived episomes replicatedextrachromosomally in the nontumorigenic 5637 bladder cell line. Thesedata suggest that the tissue tropism of viruses from which episomalconstructs are derived may predict the cell type in which episomalconstructs are active.

2. Large T Antigen Mutants

The replication activity of BKV episomes is dependent on expression ofthe BK-T. BK-T has a 75% amino acid homology to the SV40 large T antigen(SV-T) (Yang, et al., 1979), a protein having well-describedimmortalization and tumorigenic properties (Shin, et al., 1975, Proc.Natl. Acad. Sci. U.S.A., 72: 4435-4439; Christian, et al., 1987, CancerRes., 47: 6066-6073; Michalovitz, et al., 1987, J. Virol., 61:2648-2654; Hanahan, et al., 1989, Science, 246: 1265-1275; DeCaprio, etal., 1988, Cell, 54: 275-283; Chen, et al., 1990, J. Virol., 64:3350-3357; Chen, et al., 1992, Oncogene, 7: 1167-1175). Similar to SV-T,BK-T can bind to and thereby inactivate wild-type p53 and retinoblastoma(RB) tumor suppressor gene products (Mann, et al., 1984; Dyson, et al.,1990, J. Virol. 64: 1353-1356), the primary proposed mechanism by whichthese T antigens induce tumorigenic properties (DeCaprio, et al., 1988;Chen, et al., 1992). Transgenic mice expressing BK-T develop renalcarcinomas and thymoproliferative disorders (Dalrymple, et al., 1990, J.Virol., 64: 1182-1191), and BK-T can transform NIH 3T3 cells and babyrat kidney cells (Nakshatri, et al., 1988, J. Virol., 62: 4613-4621). Itis therefore possible that BKV episomal vectors containing wild-typeBK-T could confer tumorigenic properties to some nontumorigenic celllines, making such an episomal vector unsuitable for use in genetherapy, because the vector may be able to confer soft agar growth oncells in culture or induce neoplastic transformation in vivo.

However, the significant homology between SV-T and BK-T led to aspecific strategy to solve this problem. SV-T can bind to the BKV originof replication in vitro and can stimulate the replication of a plasmidcontaining the BKV origin of replication in COS cells (Ryder, et al.,1983, Virol., 129: 239-245; Deyerie, et al., 1989, J. Virol., 63:356-365). Therefore, replication-competent SV-T mutants havingsuppressed transformation properties were examined as substitutes forBK-T to promote replication of BKV episomes without transformation.

Replication-Competent, Transformation-Negative SV-T Mutants

The domain of SV-T which binds the SV40 DNA origin is separate anddistinct from the RB and p53 binding domains, as illustrated in Example6 below. Three replication competent, transformation negative SV-Tmutants are also illustrated. The first SV-T mutant is 107-T (alsoreferred to as Kl, Kalderon, et al., 1984, Virol., 139: 109-137), whichis replication competent yet nontumorigenic in several cell types(DeCaprio, et al., 1988; Chen, et al., 1990; Chen, et al., 1992;Kalderon, et al., 1984; Cherington, et al., 1988, Mol. Cell Biol., 8:1380-1384). 107-T differs from wild-type SV-T in a single base pairresulting in substitution of lysine for glutamic acid in codon 107.Codon 107 is in the RB binding domain of SV-T, and the inability of107-T to bind RB most likely accounts for its nontumorigenic properties.The DNA binding region of 107-T is intact, however, and as shown below,we have determined that 107-T can drive replication of a test plasmidcontaining the SV40 DNA origin.

The second mutant is 402-T, which has a substitution of glutamic acidfor asparagine in codon 402 (Lin, et al., 1991, J. Virol., 65:2066-2072). The 402-T point mutation is in the p53 binding domain ofSV-T, and 402-T fails to bind wild-type p53, although it appears to bindRB, and can also drive replication of the SV40 DNA origin. 402-T isnontransforming in human diploid fibroblast lines D.551 and WI-38 (Lin,et al., 1991, J. Virol., 65: 6447-6453).

Lastly, a novel SV-T mutant has been constructed which contains bothpoint mutations found in 107-T and 402-T (107/402-T). This SV-T mutantwill not bind either p53 or RB, and will have very low potential toconfer tumorigenic properties. Replication competent 107/402-T will haveparticular value.

Integrating vectors encoding these three different SV-T mutants whichhave differing abilities to bind to wild-type p53 and RB have beenprepared, and these SV-T mutant vectors have been transfected intonontumorigenic bladder cell lines. Single cell clones have beencharacterized which express the mutant SV-T molecules, yet remainnontumorigenic, and some of these clones have been shown to drivereplication of plasmids containing SV40 DNA origins. This strategy hastherefore been successful in modifying a papovavirus large T antigen foruse in episomal vectors carrying an SV40 replicon for efficientexpression of foreign genes in nontumorigenic cells.

The large T antigen mutants encoded by replication cassettes of thisinvention must be replication-competent and transformation-negative,that is they must induce DNA replication and not transform the hostcell. Trans-activation of DNA replication can be tested using Southernblot analysis of Hirt supernatant or total cellular DNA extracted fromtransient episomal transfectants, as described above.

The transforming activity of the mutant large T antigen can be testeddirectly (see, e.g., Nakshatri, et al. 1988) or cells transfected withan expression vector expressing the mutant T antigen can be tested forsoft agar cloning activity or growth in nude or SCID mice to determinewhether the mutant T antigen is transformation-negative. Alternatively,mutants may be selected based on negative binding studies with wild-typep53 and wild-type RB. One suitable assay measures binding by generatingin vitro translated mutant large T antigen protein and mixing it withauthentic wild-type p53 or RB (e.g. in vitro translated or baculovirusproduced) before immunoprecipitation with antisera to p53 or RB,respectively, to immunoprecipitate these proteins and any T antigenproteins complexed to them. Western blots of the immunoprecipitate maybe developed with antisera to large T antigen, which will detect mutantT antigens that are positive for binding.

An alternative procedure would be transfecting a population of mammaliancells expressing wild-type p53 and RB (preferably from a human cellline) with an expression vector so that the cells express the large Tmutant (as detected by, e.g., binding to antisera for T antigen). Thecells are then lysed and the lysate treated with antisera to p53 or RB.The immunoprecipitate is treated as before. This latter assay has somepotential for false-negatives if, for instance, the amount of mutant Tantigen expressed is significantly different from the amount of p53 orRB present, or if there are subtle mutations in the p53 or RB expressedby the test cell, but it more closely approximates the in vivoconditions.

A particularly preferred mutant large T antigen is the SV-T mutant107/402-T described above. Other mutants of SV-T and other papovaviruslarge T antigens have been described in the literature, and additionalmutants can be generated by well-known recombinant DNA techniques. Thesemutants will be suitable for the replication cassette of this invention,so long as they are replication-competent and transformation-negative asdetermined by the above tests.

3. Promoters

In general, replication-competent, transformation-negative papovaviruslarge T antigen will be transcriptionally regulated by eitherheterologous or homologous promoters. The heterologous promoters areusually promoters which are active in mammalian cells, such as mammalianpromoters and mammalian viral promoters. Where the episomalamplification cassette is part of an episomal expression vector for genetherapy application, the promoter will, of course, be chosen to beactive in the cell which is the target for expression of the foreigngene.

Some heterologous promoters, such as CMV immediate earlypromoter-enhancer, are not down-regulated by T antigen, therebymaximizing T antigen expression and consequently, episomal replication.This may be particularly advantageous in transient transfectionstrategies for gene therapy applications in which high level geneexpression is desirable. Alternatively, use of homologous papovavirusespromoters, which are down regulated by T antigen, may constrain runawayepisomal replication, thereby achieving controlled, stable expression.Such a promoter/T antigen/origin combination will provide high copynumber, stable episomes in transfected cells.

Alternatively, the promoter controlling expression of the mutant large Tantigen may be selected to regulate episomal replication. For example aninducible promoter (such as the metallothionen promoter) may be used,and replication of the episome will be amplified in the presence of theinducer. Alternatively, a promoter for a developmentally-controlled ortissue-specific gene (e.g., the breast specific promoter for the wheyacidic protein gene, Shoeneberger, et al., 1988, EMBO J., 7: 169-175)may be used to limit the amplification of the episome copy number tocertain cell types where that promoter is active. In gene therapy usingan episome which carries a foreign gene whose expression level isproportional to copy number, selection of the promoter controlling Tantigen expression provides a measure of therapeutic control ofexpression.

4. Vectors for Insertion of Cassettes

Broadly, the vectors into which the replication cassette of thisinvention may be inserted may be any vector that will carry thecassette, and any associated foreign genes, into mammalian cells inwhich the particular papovavirus origin and large T antigen will drivereplication of the vector. The vector, of course, will not contain anysequences that prevent replication from the papovavirus origin ofreplication in mammalian cells or prevent expression of any foreign geneinserted into the vector for gene therapy applications. Suitable vectorsinclude bacterial plasmids, which are useful as shuttle vectors toproduce large quantities of the vector containing the replicationcassette in bacterial culture for subsequent use in transfection ofmammalian cells. Other suitable vectors include well-known mammalianvectors, usually of vital origin, which are known to transfect mammaliancells, and are non-pathogenic, or of limited pathogenicity, includingdefective or mutant viruses (see, e.g., Hock, et al. 1986, Nature, 320:275-277; Sorrentino, et al. 1992, Science, 257: 99-103; Bayle, et at.1993, Human Gene Therapy, 4: 161-170; Le Gal La Salle, et al. 1993,Science, 259: 988-990; Quantin, et al. 1992, Proc. Natl. Acad. Sci.U.S.A., 89: 2581-2584; Rosenfeld, et al. 1992, Cell, 68: 143-155). Wherethe vector is a mammalian virus, it is of course important thatinsertion of foreign genes into the vital genome does not destroy viralinfectivity. Selection of a particular vector will take into account theparticular mammal and the particular cell type in which episomalamplification is desired, and the skilled worker can readily selectsuitable vectors from among many available in art. (See, e.g., Sambrook,et al., 1989, "Molecular Cloning: A Laboratory Manual"; Miller, et al.,1989, BioTechnqiues, 7: 980-990; Salmons, et al., 1993, Human GeneTherapy, 4: 129-141; Stratford-Perricaudet, et al., 1991, in "Human GeneTransfer," Cohen-Haguenauer, et al., eds., John Libbery Eurotest Ltd.219: 51-61).

III. Method of Constructing the Vector

A. Sources of Component DNA Sequences

The DNA sequences of various papovaviruses are described in theliterature, including the DNA sequences encoding the origin ofreplication, the early promoter, and the large T antigen. (See, e.g.,(for SV40) Subramanian, et al. 1977, J. Biol. Chem., 252: 355-367;Reddy, et al. 1978, Science, 200: 494-502; Fiers, et al. 1978, Nature,273: 113-120; Van Heuverswyn, et al. 1978, Eur. J. Biochem., 100: 51-60;(for BKV) Yang, et al. 1979, Science, 206: 456-461; Deyerle, et al.1989, J. Virol., 63: 356-365; (for hamster papovavirus) Delmas, et al.1985, EMBO J., 4: 1279-1286; (for JC virus) Frisque, et al. 1984, J.Virol., 51: 458-469; (for polyoma) Zhu, et al. 1984, J. Virol., 51:170-180.) Clones containing many of the sequences are contained invarious mammalian vectors available from commercial suppliers, such asStratagene, Gibco-BRL Life Technologies, United States Biochemicals, andPromega. Clones containing the complete genomic sequence for BK virus,JC virus, K virus, polyoma virus, and SV40 are available from AmericanType Culture Collection, 12301 Parklawn Drive, Rockville, Md. 20852,U.S.A. (ATCC). Clones containing promoters, bacterial origins ofreplication, and a variety of vectors are also available from thecommercial sources listed above or ATCC, as well as other sources wellknown to those skilled in the art of recombinant DNA manipulation.Specific sequences encoding particular proteins or regulatory sequencesmay be obtained from these clones using standard recombinant DNAtechniques, such as those described below. The particular foreign geneswhose expression in mammalian cells is desired, and sources forsequences encoding them, will be readily apparent to those skilled inthe art of gene therapy.

B. Recombinant: Procedures For Vector Construction

The practice of the present invention employs, unless otherwiseindicated, conventional molecular biology, microbiology, and recombinantDNA techniques within the skill of the art. Such techniques are wellknown to the skilled worker and are explained fully in the literature.See, e.g., Maniatis, Fritsch & Sambrook, "Molecular Cloning: ALaboratory Manual" (1982); "DNA Cloning: A Practical Approach," VolumesI and II (D. N. Glover, ed., 1985); "Oligonucleotide Synthesis" (M. J.Gait, ed., 1984); "Nucleic Acid Hybridization" (B. D. Hames & S. J.Higgins, eds., 1985); "Transcription and Translation" (B. D. Hames & S.J. Higgins, eds., 1984); "Animal Cell Culture" (R. I. Freshney, ed.,1986); "Immobilized Cells and Enzymes" (IRL Press, 1986); B. Perbal, "APractical Guide to Molecular Cloning" (1984), and Sambrook, et al.,"Molecular Cloning: a Laboratory Manual" (1989).

DNA segments corresponding to the papovavirus origin of replication, thepapovavirus large T antigen coding sequence and the papovavirus earlypromoter may be obtained from readily available recombinant DNAmaterials, such as those available from the ATCC, which include BKvirus, JC virus, K virus, polyoma virus, and SV40 virus. DNA segments oroligonucleotides having specific sequences can be synthesized chemicallyor isolated by one of several approaches. The basic strategies foridentifying, amplifying and isolating desired DNA sequences as well asassembling them into larger DNA molecules containing the desiredsequence domains in the desired order, are well known to those ofordinary skill in the art. See, e.g., Sambrook, et al., (1989); B.Perbal, (1984). Preferably, DNA segments corresponding to thepapovavirus origin, large T antigen and early promoter may be isolatedindividually using the polymerase chain reaction (M. A. Innis, et al.,"PCR Protocols: A Guide To Methods and Applications," Academic Press,1990). A complete sequence may be assembled from overlappingoligonucleotides prepared by standard methods and assembled into acomplete coding sequence. See, e.g., Edge (1981) Nature 292: 756;Nambair, et al. (1984) Science 223: 1299; Jay, et al. (1984) J. Biol.Chem., 259: 6311.

The assembled sequence can be cloned into any suitable vector orreplicon and maintained there in a composition which is substantiallyfree of vectors that do not contain the assembled sequence. Thisprovides a reservoir of the assembled sequence, and segments or theentire sequence can be extracted from the reservoir by excising from DNAin the reservoir material with restriction enzymes or by PCRamplification. Numerous cloning vectors are known to those of skill inthe art, and the selection of an appropriate cloning vector is a matterof choice (see, e.g., Sambrook, et al., incorporated herein byreference). The construction of vectors containing desired DNA segmentslinked by appropriate DNA sequences is accomplished by techniquessimilar to those used to construct the segments. These vectors may beconstructed to contain additional DNA segments, such as those encodingforeign genes for gene therapy, bacterial origins of replication to makeshuttle vectors (for shuttling between prokaryotic intermediate hostsand mammalian final hosts), etc.

Procedures for construction and expression of mutant proteins of definedsequence are well known in the art. A DNA sequence encoding a knownmutant of papovavirus large T antigen can be synthesized chemically orprepared from the wild-type sequence by one of several approaches,including primer extension, linker insertion and PCR (see, e.g.,Sambrook, et al.). Alternatively, additional mutants can be prepared bythese techniques having additions, deletions and substitutions in thewild-type sequence. In either case, it is preferable to test the mutantsto confirm that they are replication-competent andtransformation-negative, by the assays described above. Mutant large Tantigen protein for testing may be prepared by placing the codingsequence for the polypeptide in a vector under the control of apromoter, so that the DNA sequence is transcribed into RNA andtranslated into protein in a host cell transformed by this (expression)vector. The mutant large T antigen protein may be produced by growinghost cells transfected by an expression vector containing the codingsequence for the mutant T antigen under conditions whereby thepolypeptide is expressed. The selection of the appropriate growthconditions is within the skill of the art.

C. Intermediate Stage Vectors

Preferably the vector containing the replication cassette will alsocontain a functional bacterial origin of replication and selectionmarkers that function in bacteria (i.e., a shuttle vector). These willallow cloning of the vector in bacteria to provide a stable reservoir ofthe vector for storage and to facilitate amplification, where largequantities of the vector containing the replication cassette and anyassociated foreign genes can be recovered from bacterial culture. Theprocedures, as well as appropriate bacterial origins and selectionmarkers are well known in the art (see, e.g. Sambrook, et al.).Alternatively, mammalian viral vectors may be amplified in mammaliancell culture, using well known techniques. Appropriate procedures forstorage and standardization of preparations containing virus vectors orbacterial cells harboring shuttle plasmid vectors will be readilyapparent to those skilled in the art.

D. Functional Tests of the Vector

Vectors containing the replication cassette of this invention willroutinely be tested after they have been constructed to confirm that thevector is replication-competent and non-transforming. These tests willassure that sequences included in the vector do not interfere with thefunctioning of the replication cassette. Replication competence (i.e.,that both the mutant large T antigen and the origin of replication arefunctional) is usually tested by transfecting a population ofnon-transformed cells of the target cell type with the vector andmonitoring episomal DNA production by Southern blot. Stabletransfectants from the replication test can be further tested for softagar cloning activity or tumorigenesis in nude or SCID mice to confirmthat the vector has not transformed the cells. Southern blots of DNAfrom the stable transfectants may be used to indicate whether they haveintegrated the vector into genomic DNA or if the vector is being carriedas a stable episome.

IV. Use of the Vector

A. Therapeutic Use

Vectors for use in gene therapy are constructed by inserting thereplication cassette of this invention into a suitable mammalian vectoralong with the foreign gene whose expression is desired, using standardrecombinant DNA techniques as described above to produce an episomalexpression vector. Cells are then transfected with these episomalexpression vectors under conditions that maintain cell viability and thevectors replicate episomally in the cells. The episomal expressionvectors may be administered to patients in a variety of ways.

In one embodiment cells are transfected with the episomal expressionvector in vitro. Usually, appropriate cells are obtained from thepatient, for instance peripheral blood monocytes from a blood sample,and these cells are transfected with the episomal expression vectorbefore being re-introduced into the patient. Alternatively, stem cellsin a population of the patient's cells are cultured to provide a largecell population, compatible with the patient, and the cells aretransfected with the episomal expression vector in culture. Then thetransfected cell population is re-introduced into the patient.

In another embodiment, the episomal expression vector is based on amammalian virus which infects the patient mammal, containing areplication cassette according to this invention and a foreign gene. Theviral episomal expression vector is administered to the patient, whereit infects the patient's cells, and the episomal expression vector thenreplicates episomally in the cells.

In another embodiment, the episomal expression vector is introduced intothe patient mammal in conjunction with liposomal or receptor-mediateddelivery systems (see Felgner, et at., 1987, Proc. Natl. Acad. SciU.S.A. 84: 7413-7417 and Zhu, et al., 1993, Science, 261: 209-211,incorporated herein by reference). Once the patients' cells aretransfected in vivo, the episomal expression vector will replicateextrachromasomally.

Expression of the foreign gene may occur once the cell has beentransfected by the episomal expression vector. Usually the foreign genewill be expressed constitutively, and the level of expression will becontrolled by the copy number of the episome. In another embodiment,expression of the foreign gene will be under control of a promoter thancan be up- or down-regulated in a manner described above for expressionof the mutant large T antigen. Selection of suitable promoters forcontrol of the foreign gene will be apparent to the skilled worker,based on the desired clinical result.

While any gene that can be expressed in a mammalian cell may beincorporated into a transfection vector as the foreign gene according tothis invention, preferred genes will be those whose expression in atarget cell population will counter-act a disease process. For example,an episomal gene therapy vector could be used to target the immunesystem to kill cancer cells in vivo. Tumor cell lines transfected withcytokine cDNA have been successfully used as cancer vaccines (Connor, etal., 1993, J. Exp. Med., 177: 1127-1134; Golumbek, et al., 1991,Science, 254: 713; Porgador, et al., 1992, Cancer Res., 52: 3678; Aoki,et al., 1992, Proc. Natl. Acad. Sci. U.S.A., 89: 3850) and transfectionof tumor cells in vivo with appropriate episomal vectors will enhancetumor kill, since episomal replication in the tumor cell willefficiently produce the desired high local concentration of cytokines,thereby stimulating immune effector cells. One such example isintroduction of episomal expression vectors encoding interleukin-2 intobladder cancer cells in vivo via instillation of liposome/DNA complexesdirectly into the bladder tureen. Another example is transfection oflung cancer cells in vivo with interleukin-6 via inhalation ofaerosolized liposome/DNA complexes (see Stribling, et al., 1992, Proc.Natl. Acad. Sci. U.S.A., 89: 11277-11281, for method using non-episomalvectors). Other gene therapy approaches to kill cancer cells includeexpression of genes conferring drug susceptibility, such as transfectionwith herpes simplex thymidine kinase encoding vectors followed byganciclovir treatment. (Culver, et al., 1992, Science, 256: 1550-1552used integrating vectors. Replacing the integrating vector with anepisomal expression vector will enhance the level of susceptibilityconferring enzyme.) Other foreign gene sequences whose expression by apatient's cells would counter-act a disease process will be apparent tothose skilled in the art.

The presence of multiple copies of papovavirus-based episomes mayincrease expression of encoded genes compared to retroviral vectors,since only one to several copies of the retrovirus integrate per cell.Additionally, episomal DNA would be free of positional effects that mayresult in decreased expression from integrated vectors. The high levelof transcription produced by episomal vectors of this invention may beparticularly useful in antisense experiments, because high-levelexpression of antisense transcripts may be necessary to decreasetranslation of overexpressed target mRNA (Whitesell, et al., 1991, Mol.Cell. Biol., 11: 1360-1371).

The persistence of BKV episomes in pRP-cneoX/HT-1376 transfectants afterwithdrawal of selection pressure suggests that these vectors may bemaintained for a reasonable period of time in human tissues. Even atransient period episomal replication may be sufficient for effectiveuse of papovavirus episomes to treat patients with cancer. For example,wild-type anti-oncogenes capable of inducing apoptosis, such as p53(Baker, et al., 1990, Science, 249: 912-915; Shaw, et al., 1991, Proc.Natl. Acad. Sci. U.S.A., 89: 4495-4499), may need to be expressed foronly a short period of time to kill transfected tumor cells. Similarly,transient expression of genes encoding susceptibility factors tochemotherapeutic agents may be effective in killing tumor cells, as hasrecently been demonstrated for herpes simplex thymidine kinase followedby gancyclovir treatment, and cytosine deaminase followed by5'-fluorocytosine treatment (Culver, et al., 1992, Science, 256:1550-1552; Muller, et al., 1992, Proc. Natl. Acad. Sci. U.S.A., 89:33-37). Furthermore, transient expression of cytokines, such asinterleukin-4, may be effective in modulating the immune system toeliminate tumor cells (Golumbek, et al., 1991, Science, 254: 713-716).

The episomal vector material is generally produced by culture ofrecombinant or transfected cells and formulated in a pharmacologicallyacceptable solution or suspension, which is usually aphysiologically-compatible aqueous solution, or in coated tablets,tablets, capsules, suppositories, inhalation aerosols, or ampules, asdescribed in the art, for example in U.S. Pat. No. 4,446,128,incorporated herein by reference. Administration may be any suitableroute, including oral, rectal, intranasal or by intravesicular (e.g.bladder) instillation or injection where injection may be, for example,transdermal, subcutaneous, intramuscular or intravenous.

The vector-containing composition is administered to a mammal in anamount sufficient to transfect a substantial portion of the target cellsof the mammal. Determination of the amount will involve consideration ofinfectivity of the vector, transfection efficiency in vitro, immuneresponse of the patient, etc. A typical initial dose for administrationwould be 10-1000 micrograms when administered intravenously,intramuscularly, subcutaneously, intravesicularly, or in inhalationaerosol, 100 to 1000 micrograms by mouth, or 10⁵ to 10¹⁰ plaque formingunits of a recombinant vector, although this amount may be adjusted by aclinician doing the administration as commonly occurs in theadministration of other pharmacological agents. A single administrationmay usually be sufficient to produce a therapeutic effect, but multipleadministrations may be necessary to assure continued response over asubstantial period of time.

Further description of suitable methods of formulation andadministration according to this invention may be found in U.S. Pat.Nos. 4,592,002 and 4,920,209, incorporated herein by reference.

B. Alternative Uses for Claimed Compositions

Episomal expression vectors according to this invention also have invitro uses. For example, episomal expression vectors may be used toenhance production of proteins that are produced in mammalian cellculture, perhaps because they must be post-translationally glycosylated(e.g., Factor VIII). Transfecting the production cell population with anepisomal expression vector containing a replication cassette prepared asdescribed above and a foreign gene encoding the desired protein willincrease the expression level of the desired protein, as episomalamplification leads to high copy number of the foreign gene in theproduction cells.

The episomal expression vectors of this invention may be used in studiesto identify potentially novel dominant oncogenes and/or anti-oncogenesthat are involved in tumor progression. By constitutively expressingcDNA clones at high levels, this approach may identify new genes orgenes marking biological pathways that currently have not been shown tobe involved in tumorigenesis. Moreover, this assay system will permitanti-sense cDNA library screening for tumor suppressor genes, greatlysimplifying the currently labor intensive methods required to identifymembers of this class of genes.

This may be accomplished, for instance, by transfecting nontumorigenicbladder cell lines that do not clone in soft agar with cDNA derived fromtumorigenic, anchorage independent cell lines, and phenotypicallyscreening the transfectants for the ability to grow in soft agar. ThecDNAs responsible for inducing nontumorigenic cells to clone in softagar will then be identified by shuttling the episomal vector frombladder cell transfectants to bacteria. Second generation transfectionstudies will subsequently confirm the ability of candidate dominantoncogenes to fully transform nontumorigenic bladder cells. In a similarfashion, cDNA libraries from nontransformed cells screened in ananti-sense orientation could theoretically identify anti-oncogenes.

EXAMPLES

In order to facilitate a more complete understanding of the invention, anumber of Examples are provided below. However, the scope of theinvention is not limited to specific embodiments disclosed in theseExamples, which are for purposes of illustration only.

Example 1

BKV episomal plasmids can stably replicate in HT-1376 cells.

Plasmids

pRP-cCATX and pRP-cncoX are BKV episomal plasmids that contain a 3.2-kbfragment of BKV encoding the origin of DNA replication and the large Tantigen (Grossi, et al., 1988). pRP-cCATX encodes the chloramphenicolacetyltransferase (CAT) gene driven by the SV40 early promoter, whereaspRP-cncox encodes the neomycin resistance gene [phosphotransferaseAPH(3')II from transporon Th5] driven by the SV40 early promoter.pSV2CAT/220.2 is a derivative of pSV2CAT containing the EBV episomalelement (Haver, et al., 1989). pSV2NEO encodes the neomycin resistancegene driven by the SV40 early promoter (Southern and Berg, 1982).pSV2CAT encodes the CAT gene transcriptionally regulated by the SV40early promoter, and pSVOCAT is a derivative of pSV2CAT lacking the SV40early promoter (Gorman, et al., 1982).

To evaluate if a BKV episomal plasmid can stably replicate in bladdercarcinoma cells, HT-1376 cells were transfected with pRP-cneoX, aderivative of pSV2NEO containing a 3.2 kb episomal element consisting ofthe BKV origin of DNA replication and the BKV large T antigen (Grossi,et al., 1988, Arch. Virol., 102: 275-283).

Transfection and Selection

A total of 1.5×10⁶ cells in 60-mm dishes were transfected using 10 μg ofplasmid DNA and 40 μg of lipofectin in 3 ml of Optimem (Gibco-BethesdaResearch Labs, Gaithersburg, Md.). Following 6 hours of incubation, DMEMwas added with supplemental fetal calf serum to obtain a finalconcentration of 10%. Two days after transfection, cells weretrypsinized and seeded in six-well plates, and 24 hours later 200 μg/mlG418 was added to the media to initiate selection.

Southern Blots

DNA from transfected cells was evaluated by Southern blot after 71 daysof G418 selection, and the Southern blots were probed with ³² P-labelledpRP-cneoX. Low-molecular-weight DNA (Hirt supernatant DNA) was preparedfrom HT-1376 transfectants as described by Hin, 1967, J. Mol. Biol., 26:365-369). Total cellular DNA was removed from CsC1 gradients andpurified as previously described (Davis, et al., 1986, "Basic Methods InMolecular Biology", Elsevier Science Publishing, N.Y., pp. 130-135).Hirt supernatant and digested total cellular DNAs were electrophoresedin 0.7% agarose gels, transferred to Nytran membranes (Schleicher &Scheull, Keene, N.H.), hybridized to ³² P-labeled random primed probes,and washed to a final stringency of 0.2× saline sodium citrate(SSC)/1.0% sodium dodecyl sulfate (SDS) at 65° C.

A. Episomal Replication in Stable Transfectants

Low molecular weight DNA (Hirt supernatant DNA) derived from thesestable transfectants was subjected to Southern blot analysis, and thedata, presented below in FIG. 1, Panel A, shows episomal plasmids.Plasmid forms II (nicked circular) and III (supercoiled) are evident inlane 1, indicating that free plasmid DNA is present in thesetransfectants. The pRP-cneoX episome in these Hirt supernatants is thesame size as plasmid controls, indicating no detectable rearrangementsor internal deletions of the episome as it is passaged in HT-1376 cells.

To confirm that this plasmid DNA is newly replicated episomal DNA, lanes2 and 3 show Hirt supernatant DNA was digested with DpnI and MboI,respectively (Pipas, et al., 1983, Mol. Cell. Biol., 3: 203-213). DpnIwill cleave the GATC recognition site when both adenine bases aremethylated, a feature of plasmid DNA synthesized in DNA adeninemethylase (DAM) positive bacteria (input DNA). In contrast, MboI willcleave the GATC recognition sequence when adenine bases are notmethylated. Since human cells lack the DAM enzyme, MboI digestion ofHirt supernatant DNA indicates that the episome replicatedextrachromosomally. The lack of restriction fragments following DpnIdigestion and the complete cleavage of Hirt supernatant DNA followingMboI digestion confirms that the plasmid DNA present in thesetransfectants is newly replicated, episomal DNA.

B. There is a high copy number of BKV episomes in HT-1376 transfectants.To determine the copy number of episomes per cell, BamHI-digested Hirtsupernatant DNA and a standard curve consisting of increasing amounts(50-400 pg) of BamHI-digested, linearized pRP-cneoX plasmid wasevaluated by Southern blot analysis (FIG. 1, Panel A, lanes 4-9).Densitometric analysis of these bands indicates that there isapproximately 500 pg of pRP-cneoX per 3×10⁵ cells, or approximately 150copies of the episome per cell. This copy number is higher than reportedfor most other episomal vectors. For example, a typical copy number ofEpstein-Barr virus-based episomal vectors in lymphoid cells isapproximately 10 to 50 per cell (Yates, et al., 1985). The high copynumber of BKV episomes in HT-1376 cells suggests that the steady statelevel for transcription of genes encoded by such a vector is likely tobe very high, and that this vector will be efficiently transferred tothe progeny of these bladder transfectants during cellular division.Both of these possibilities were evaluated in the experiments whichfollow.

C. There is no evidence that pRP-cneoX integrates into HT-1376 genomicDNA. To evaluate if pRP-cneoX also integrates into HT-1376 DNA, totalcellular DNA from HT-1376 transfectants was digested with BamHI andevaluated by Southern blot analysis (FIG. 1, Panel B, lane 1). A singleband of 9.9 kb, identical in size to BamHI-digested pRP-cneoX plasmidcontrol (lane 2), is consistent with linearized episomal plasmid.Although a 9.9 kb band might also be due to tandem copies of integratedpRP-cneoX plasmid, the absence of other restriction fragments in thisanalysis indicates that the frequency of integration of the pRP-cneoXepisome is very low, beneath the limit of resolution of this assay. Thisfinding is important, since a low frequency of integration of BKVepisomes will limit chance insertional activation of proto-oncogenes orinsertional inactivation of tumor suppressor genes.

Example 2

The steady state level of transcription of the neomycin resistance genein pRP-cneoX transfectants is 20-fold higher than in pSV2NEOtransfectants.

In order to evaluate the potential advantages of utilizing a BKVepisomal vector compared to standard plasmid vectors, HT-1376 cells weretransacted with pSV2NEO, a plasmid that will be unable to replicateextrachromosomally in HT-1376 cells in the absence of exogenous large Tantigen. Following transfection, cells were selected in neomycin, and 5to 10 clones were combined in each of 3 different pools (a, b, c).Southern analysis of genomic DNA derived from these pools indicated thatthere were approximately 5 copies of pSV2NEO per cell (data not shown).

To evaluate whether BKV episomal expression vectors can produce highlevels of transcription, the steady state level of neomycin resistancegene mRNA from pRP-cneoX and pSV2NEO HT-1376 transfectants were comparedby Northern blot analysis. In both plasmids, transcription of theneomycin resistance gene is regulated by the SV40 early promoter.

Northern Blots

Total cellular RNA was prepared by the CsC1 isothiocyanate method ofChirgwin, et al. (1979, Biochem, 18: 5294-5299). Twenty micrograms oftotal cellular RNA were electrophoresed in 1% agarose formaldehyde gels,transferred to Nytran membranes, hybridized to ³² P-labeled randomprimed probes, and washed to a final stringency of 0.1× SSC/1.0% SDS at65° C., as previously described (Cooper, et al., 1990, Cell GrowthDifferen., 1: 149-159).

In FIG. 2, Panel A, 20 μg of total cellular RNA from pRP-cneoX (lane 1)and pSV2NEO (pool a, lane 2; pool b, lane 3; pool c, lane 4)transfectants were probed with a radiolabelled BamHI/HindIII fragment ofpSV2NEO encoding the neomycin resistance gene. To confirm thatapproximately equal amounts of mRNA were loaded in each lane, this blotwas reprobed with a radiolabelled beta-actin probe (FIG. 2, Panel B).

Densitometric analysis of these data indicates that there areapproximately 20-fold higher levels of steady state expression of theneomycin resistance gene in the pRP-cneoX episomal transfectantscompared to the pSV2NEO transfectants. This difference is presumably duein part to the higher copy number of pRP-cneoX (150 copies) compared topSV2NEO (5 copies) in HT-1376 transfectants. These data indicate thatBKV episomal expression vectors can achieve substantially higher levelsof transcription of a transfected gene than plasmid or retroviralvectors that depend on integration for stable expression.

Example 3.

BKV episomal vectors are efficiently transferred to the progeny ofbladder transfectants during cellular division.

In order to effectively use an episomal expression vector for cDNAlibrary screening, the episome must be efficiently transferred from onetransfected cell to its progeny during cell division. A high copy numberof episomes per cell may be predictive of efficient vertical transfersince, in this circumstance, it would be unlikely that all episomeswould partition to a single daughter cell. Since the parent cell line,HT-1376, clones in soft agar, it was possible to directly evaluate thevertical transfer efficiency of pRP-cneoX in HT-1376 transfectants byplating these cells in soft agar in the presence or absence of neomycin.As a positive control for efficient vertical transfer, pSV2NEO HT-1376transfectants (pool b), in which the neomycin resistance gene isintegrated into HT-1376 DNA, was plated in soft agar in the presence orabsence of neomycin.

These results are presented below in Table 1. The parent cell line,HT-1376, clones in soft agar in the absence of neomycin with anefficiency of 0.83%, and does not clone in 200 pg/ml of neomycin, aconcentration previously shown to kill these cells after 14 days ofincubation. The ratio of soft agar cloning efficiencies with and withoutneomycin for both types of transfectants is essentially identical,demonstrating efficient transfer of the episome during cell division. Inaddition, there was no difference in the size of soft agar colonies ofHT-1376 pRP-cneoX transfectants grown in the absence or presence ofneomycin (data not shown), further evidence in support of efficientvertical transfer of BKV episomes in these cells.

                  TABLE 1                                                         ______________________________________                                        Vertical transfer efficiency of pRP-cneoX in HT-1376                          bladder cells.                                                                SOFT AGAR CLONING EFFICIENCY*                                                           NO                                                                            NEOMYCIN  200 μg/ML NEOMYCIN                                     ______________________________________                                        ht-1376     0.83 +/- 0.09                                                                             0                                                     ht-1376/pSV2NEO                                                                           0.75 +/- .0.01                                                                            0.65 +/ 0.16                                          ht-1376/pRP-cneoX                                                                         0.82 +/- 0.07                                                                             0.59 +/- 0.17                                         ______________________________________                                         *Cloning efficiency is expressed as the number of soft agar colonies          divided by the number of cells plated, tabulated as a percentage. 10.sup.     cells were plated per dish and colonies were scored after 3 weeks of          growth. Values are the mean of triplicate determinations +/- standard         deviation.                                                               

Example 4.

BKV episomes persist in bladder transfectants after withdrawal ofselection pressure.

The high copy number of pRP-cneoX in HT-1376 transfectants and theefficient vertical transfer of this episome to the progeny of thesetransfectants raised the possibility that pRP-cneoX may be maintained inthese cells for several weeks or months without selection pressure. Toevaluate the persistence of pRP-cneoX in HT-1376 cells in the absence ofselection pressure, these transfectants were grown in complete mediawithout G418, and at various times Hirt supernatant DNA was prepared forSouthern analysis. In FIG. 3, lanes 3-6, Hirt supernatant DNA isanalyzed from transfectants grown in the absence of G418 for 16, 34, 47,and 64 days, respectively. The sample in lane 2, from cells cultured inthe presence of G418 after 122 days of selection, coincides with the 34day time point during the course of G418 withdrawal, serving as areference for comparison.

In panel A, this blot was probed with radiolabelled pRP-cneoX. Theepisome copy number is maintained at essentially unreduced levelsfollowing 16 days of withdrawal of G418, and then appears to transientlyfall to approximately 10% of the control level by 34 days of selection.Unexpectedly, the episome copy number then increases back to controllevels by 64 days of G418 withdrawal.

To evaluate if differences in episomal copy number are due to randominefficiencies in preparation of Hirt supernatant DNA, this blot wasrehybridized to a probe for mitochondrial DNA (panel B). The essentiallyequal amount of mitochondrial DNA in each Hirt supernatant demonstratesthat extracts from comparable numbers of cells were loaded in each lane,and that the copy number of pRP-cneoX is indeed maintained at highlevels after 64 days of growth in the absence of selection pressure.These data indicate that brief periods of growth of bladder celltransfectants in the absence of G418 will be unlikely to result in lossof episomal plasmid DNA.

The finding that BKV-derived episomes are maintained at high copynumbers after 2 months of growth in the absence of G418 wasunanticipated, since EBV and SV40-derived episomes are usually lost fromstable transfectants after 2-4 weeks of growth in the absence ofselection pressures (Yates, et al., 1984; Hamber, et al. 1988;Chittenden, et al., 1991). To confirm further that pRP-cneoX ismaintained in HT-1376 cells in the absence of G418, we evaluatedexpression of the neomycin resistance gene during this time course (FIG.4A). Comparable to the episomal copy number (FIG. 3A), we observed atransient fall in neomycin resistance gene expression followed by returnto essentially control levels by 64 days of G418 withdrawal. Equivalentloading of RNA in each lane is demonstrated by rehybridizing this blotwith a probe for β-actin (FIG. 4B). These data strongly argue thatBKV-derived episomes can be maintained at a high copy number in bladdercarcinoma cells in the absence of selection pressure.

Example 5.

BKV episomal vectors can be shuttled between HT-1376 bladder celltransfectants and bacteria.

An important advantage of episomal expression vectors compared tostandard plasmid or viral constructs is the ability to shuttle theepisome from stable transfectants into competent bacteria. Hirtsupernatant DNA from HT-1376/pRP-cneoX stable transfectants was used toelectroporate DH10B E. coli. Of 12 minipreps analyzed, 10 hadunrearranged episomal plasmid (data not shown), consistent with thefindings presented in FIG. 1. For the two colonies having minorrearrangements, it is unclear whether these changes occurred in thebladder cell transfectants or during passage in bacteria.

Example 6.

Development of a replication-competent, transformation-negative hybridSV40/BKV-derived episomal expression system.

The BKV episomal vectors used in Examples 1-5 contained a 3.2 kbfragment of BKV including the origin of DNA replication and the BKVlarge T antigen (BK-T) transcriptionally regulated by the BKV earlypromoter. It is expected that BK-T would induce soft agar growth basedon its ability to complex wild-type p53 and RB (Mann, et al., 1984;Dyson, et al., 1990). As described below, this vector system wasmodified by substituting replication-competent, transformation-negativeSV40 large T antigen mutants for BK-T. This strategy has beensuccessful, and clones of bladder cell transfectants expressing highlevels of SV-T mutant protein remain nontumorigenic and induce plasmidscontaining SV40 or BKV DNA origins to replicate extrachromosomally.

Wild-type SV-T and each SV-T mutant have been subcloned into the pRc/CMVexpression vector (Invitrogen). In this vector, expression of SV-T isregulated by the efficient CMV promoter-enhancer which is active in allbladder cell lines tested. Additionally, lack of down-regulation by SV-Tprotein makes the CMV promoter-enhancer a good choice for these studies.Wild-type and mutant SV40 large T antigen cDNA were subcloned into themultiple cloning site of the CMV promoter-enhancer transcriptionalcassette in the pRC/CMV expression vector (Invitrogen) using a two-partstrategy. First, pRC/CMV.T and pRC/CMV. 107-T were constructed. cDNAsencoding wild-type SV40 large T antigen and the 107-T (K1) mutant(Kalderon, et al.) were initially available as subcloned fragments inthe unique BamHI site of the pSG5 vector. These vectors were digestedwith BamHI, fragments containing T antigen cDNA were gel purified, and3' termini were filled in using the Klenow fragment of DNA polymerase 1.Phosphorylated XbaI linkers were added, followed by XbaI digestion andgel purification. T antigen cDNA clones were then ligated intoXbaI-digested, calf intestine alkaline phosphatase-treated pRc/CMV.Orientation of the T antigen cDNA clones was determined by digestionwith XmnI and PstI. Secondly, pRC/CMV.T and pRC/CMV.107-T were modifiedto produce pRC/CMV.402-T and pRC/CMV.107/402-T. A clone of SV40 encodingthe codon 402 aspartic acid to glutamic acid mutation in SV40 large Tantigen was obtained from the laboratory of Dr. D. Simmons. This clonewas digested with HpaI, and a 1067 base pair C-terminal fragment of Tantigen was gel purified. Similarly, pRC/CMV.T and pRC/CMV-107-T weredigested with HpaI, and the large 6.4 kb fragments were gel purified.The C-terminal HpaI fragment from 402-T was then ligated with calfintestine alkaline phosphatase-treated parent vectors to producepRC/CMV.402-T and pRC/CMV.107/402-T. Orientation of the T antigen cDNAclones was determined by digestion with AlwNI.

Partial DNA sequence analysis of these constructs confirms that theseSV-T mutants do indeed contain the predicted point mutations. FIG. 5shows the location of point mutations in replication competent,transformation negative SV40 large T antigen (SV-T) mutants. The p53 andRB binding characteristics of 107/402-T, indicated in parenthesis, arepredicted results.

A. Expression of large T antigen mutants by nontumorigenic 5637 cellline. Because the 5637 cell line is nontumorigenic and has mutations inboth p53 and RB, it seemed likely that expression of either wild-type ormutant SV-T protein would not induce tumorigenic properties. 5637 wastherefore chosen for initial transfection studies. 5637 was transfectedwith pRc/CMV.SV-T and pRc/CMV.107-T using the lipofection method(Felgner, et al., 1987, Proc. Natl. Acad. Sci. U.S.A., 84: 7413-7417),stable transfectants were selected in G418, and single cell clones wereisolated using cloning cylinders. Shown in FIG. 6 below is a Westernblot analysis of representative single cell clones for SV-T and 107-Texpression. A protein of 94 kD is detected in these studies, identicalin size to T antigen produced in COS-7 control cells. All three SV-Tclones have moderate levels of T antigen expression (lanes 1-3). Threeout of 4 107-T clones had high levels of T antigen expression (lanes 4,5, and 7), with clone C10 having no detectable T antigen expression(lane 6). Expression of mutant large T antigen was similarly observedfor 5637 cells transfected with pRc/CMV.402-T and pRc/CMV.107/402-T.

B. Expression of mutant large T antigen does not induce tumorigenicproperties in susceptible cells. Clones of SV-T and 107-T 5637transfectants have been evaluated for the ability to grow in soft agar,to form foci in tissue culture, and to form tumors in the flanks of nudemice. Three 5637 clones were chosen for these initial studies (see FIG.6): C4 (SV-T expressor); C10 (107-T non-expressor); and E1 (107-Texpressor).

Soft Agar Cloning

Transformed cells were trypsinized and then passed through a 30-micronnylon filter (Tetko, Lancaster, N.Y.) to achieve a single cellsuspension. The bottom layer of agar consisted of low glucose DMEMsupplemented with 10% fetal calf serum and 0.6% Seaplaque agarose (FMCBioProducts, Rockland, Me.). The top layer contained serial dilutions ofcells ranging between 10³ and 10⁶ in low-glucose DMEM supplemented with10% fetal calf serum and 0.3% Seaplaque agarose. Aggregates of cellsgreater than 125 μm in diameter (˜50 cells) were scored as colonies, anddishes were observed for at least 1 month after plating. These dam aresummarize below in Table 2.

The parent nontumorigenic 5637 cell line does not clone in soft agar,form foci in tissue culture, and only 1/12 nude mice inoculated formed atumor after a prolonged latency period. The moderate SV-T expressor, C4,remains nontumorigenic in nude mice and does not clone in soft agar.Interestingly, this clone forms microscopic foci, although does not formmacroscopic foci. As expected, clone C10, the 107-T non-expressor, isnontumorigenic with an identical profile as the parental 5637 cell line.The high level 107-T expressor, E1, has an identical profile as C4, withformation of microscopic foci, but no growth in soft agar or in nudemice. These data demonstrate that we have successfully expressed Tantigen protein in nontumorigenic bladder carcinoma cells withoutinducing anchorage independence or tumor formation in nude mice.

                  TABLE 2                                                         ______________________________________                                        5637 bladder carcinoma cells transfected with wild-type (SV-T)                or mutant (107-T) SV40 large T antigens remain                                nontumorigenic.                                                               ANTI-                SA             LATENCY                                   GEN         FOCUS    (%)    TUMORS  (WKS)                                     ______________________________________                                        Parental                                                                              -       -        0     1/12   12                                      SV-T C4 +       +/-      0    0/6     --                                      107-T C10                                                                             -       -        0    0/6     --                                      107-T E1                                                                              ++      +/-      0    0/6     --                                      ______________________________________                                         T ANTIGEN, level of T antigen expression (- none, + moderate, ++ high);       FOCUS, focus formation (- none, +/- microscopic foci); SA, soft agar          growth; TUMORS, # tumors/total number of nude mice inoculated; LATENCY,       time (weeks) to achieve tumor volume >250 mm.sup.3                       

Expression of wild-type SV40 T antigen will confer soft agar growth in asuitable recipient cell line. Since T24 cell line has wild-type RB (andmutant p53), expression of SV-T or 402-T is expected to inactivate RBprotein. We have characterized tumorigenic properties in clones of T24cells expressing moderate levels of SV-T (A7) and 402-T (G1). Whereasthe parent T24 cell line does not clone in soft agar, A7 and G1 havecloning efficiencies of 0.10%±0.02% (se) and 0.12%±0.04%, respectively.These findings indicate that the T antigen proteins expressed in thesetransfectants are biologically functional molecules retainingtransformation properties. In contrast, a single cell clone of T24expressing high levels of 107/402-T does not grow in soft agar.

C. 107-T can drive replication of the SV40 DNA origin. A biologicalfunction expected for SV-T and 107-T is the ability to drive replicationof the SV40 DNA origin. To evaluate replication activity of 107-T, 5637clones C10 and E1 were transfected with pSV2CAT, a plasmid containingthe SV40 DNA origin. Four days after transfection, Hirt supernatant DNAwas prepared and evaluated for evidence of episomal replication, asillustrated in FIG. 7.

Extrachromosomal replication can be assayed by determining if Hirtsupernatant DNA is partially resistant to digestion by DpnI. Whereasplasmid DNA prepared in DNA adenine methylase positive bacteria aremethylated at adenine nucleotides at the sequence GATC, mammalian cellslack this enzyme, and hence human DNA is resistant to digestion by DpnI.As observed in FIG. 7, Hirt DNA prepared from the C10 5637 clone (107-Tnon-expressor) fails to support episomal replication, since pSV2CAT isefficiently digested by DpnI. In contrast, Hirt DNA prepared from the E15637 clone (107-T expressor) is largely resistant to digestion by DpnI,indicating that pSV2CAT is replicating extrachromosomally in thesetransfectants. These data demonstrate that SV40 large T antigensexpressed in bladder cell transfectants are biologically functionalmolecules, possessing replication activity.

It will be understood that while the invention has been described inconjunction with specific embodiments thereof, the foregoing descriptionand examples are intended to illustrate, but not limit the scope of theinvention. Other aspects, advantages and modifications will be apparentto those skilled in the art to which the invention pertains, and theseaspects and modifications are within the scope of the invention, whichis limited only by the appended claims.

I claim:
 1. A method of expressing a foreign gene in a mammalian cellcomprising: transfecting the mammalian cell with areplication-competent, transformation-negative vector comprising atleast one papovavirus origin of replication, a first DNA sequenceencoding a mutant form of papovavirus large T antigen which contains areplication-competent binding site for said origin of replication andwhich is negative for binding to and to retinoblastoma tumor suppressorgene product due to a mutation in a codon in the p53 binding domain ofsaid large T antigen and a mutation in a codon in the RB binding domainof said large T antigen, said DNA sequence being operatively linked to afirst promoter which is functional in said mammalian cell, and a secondDNA sequence encoding the foreign gene operatively linked to a secondpromoter which is functional in the mammalian cell, whereby thetransfected cell subsequently expresses the foreign gene.
 2. The methodof claim 1, wherein the papovavirus origin of replication is a BK virusorigin of replication or a SV40 origin of replication.
 3. The method ofclaim 2, wherein the mutant form of large T antigen contains areplication competent binding site for both the BK virus origin ofreplication and the SV40 origin of replication.
 4. The method of claim1, wherein the replication-competent, transformation-negative vectorfurther comprises a bacterial origin of replication.
 5. The method ofclaim 1, wherein the first promoter is inducible.
 6. The method of claim1, wherein the first promoter is constitutive.
 7. The method of claim 1,wherein the first promoter is under hormonal control.
 8. A DNA moleculecomprising:a DNA sequence encoding a mutant form of SV40 large T antigenwhich (a) contains a replication-competent binding site for SV40 originof replication and (b) is negative for binding to wild-type p53 and toretinoblastoma tumor suppressor gene product due to a mutation in acodon in the p53 binding domain of said large T antigen and a mutationin a codon in the RB binding domain of said large T antigen.
 9. The DNAmolecule of claim 8, wherein residue 107 of the mutant SV40 large Tantigen is lysine and residue 402 is glutamic acid.
 10. The DNA moleculeof claim 8, wherein the mutant form of SV40 large T antigen alsocontains a replication-competent binding site for a BK virus origin ofreplication.
 11. A DNA molecule comprising:a DNA sequence encoding amutant form of SV40 large T antigen which (a) activates DNA replicationby binding to BK virus origin of replication and (b) is negative forbinding to wild-type p53 and to retinoblastoma tumor suppressor geneproduct due to a mutation in a codon in the p53 binding domain of saidlarge T antigen and a mutation in a codon in the RB binding domain ofsaid large T antigen.
 12. The method of claim 1 wherein said large Tantigen is SV40 large T antigen and residue 107 of the mutant SV40 largeT antigen is lysine and residue 402 is glutamic acid.
 13. A mammalianvector which is non-pathogenic, replication-competent andtransformation-negative, said vector comprising:at least one papovavirusorigin of replication; and a DNA sequence encoding a mutant form ofpapovavirus large T antigen which (a) contains a replication-competentbinding site for said origin of replication and which (b) is negativefor binding to wild-type p53 and to retinoblastoma tumor suppressor geneproducts due to a mutation in a codon in the p53 binding domain of saidlarge T antigen and a mutation in a codon in the RB binding domain ofsaid large T antigen, said DNA sequence being operatively linked to apromoter.
 14. The vector of claim 13 wherein said large T antigen isSV40 large T antigen and residue 107 of the mutant SV40 large T antigenis lysine and residue 402 is glutamic acid.
 15. The vector of claim 13,wherein the papovavirus origin of replication is a BK virus origin ofreplication or a SV40 origin of replication.
 16. The vector of claim 15,wherein the mutant form of large T antigen contains a replicationcompetent binding site for both the BK virus origin or replication andthe SV40 origin of replication.
 17. The vector of claim 13, furthercomprising a bacterial origin of replication.
 18. The vector of claim13, wherein the promoter is inducible.
 19. The vector of claim 13,wherein the promoter is constitutive.
 20. The vector of claim 13,wherein the promoter is under hormonal control.